The opera is drama in which music is a key factor, as it includes expressive melodies for voices with orchestral accompaniment (recitativo, aria, chorus) and orchestral preludes and interludes.
Opera is one of the leading forms that combine music and theatre practice. It is very complex, involving many different arts: music (instrumental and vocal), drama, poetry, acting, dance, Scenic design, costumes, etc. Factors like high cost of production and also technical difficulties in performing contributed to the creation of a "status symbol" concerning opera artists. Because opera was very popular many aspiring composers and singers were attracted.
Most operas have been composed by specialists concerning the musical form. Only a few composers worked on opera equally along with other genres, for example: Monteverdi, Handel, Mozart, R. Strauss, Berlioz, Prokofiev, Britten and Stravinsky.
Other composers did not try it at all, or they did, however with no worth mentioning success.
As with all art forms, opera was based on a number of specific conventions. The most important of these is that characters express their thoughts and feelings through songs and not so much through speech.
Because it takes more time to develop ideas through music than through words, usually it is used one (or both) of the following techniques
1) The stretching of the story and, therefore, the slow rising and falling action (as in musical dramas of Wagner).
2) Alternating periods of action and repose. The action may be transferred to recitative, (with, as less as possible music) and it occasionally may be interrupted by musical "acts" (arias, ensembles, ballets, etc.).
According to Willi Apel, these characteristics, like the slow progress of the story and the frequent interruptions, would cause a serious flaw to a spoken theatre work , however they are necessary for the opera.
From these basic necessities arose many of the opera's features and perhaps people who are not familiar with the these nowadays find it difficult to accept. Such features are the frequent repetition of text, lengthy emotional scenes , the last words before his/her death, etc.
Willi Apel: Harvard dictionary of music, second edition, eighth printing